In recent years, for the purpose of preventing global warming, the reduction of carbon dioxide has come to be a critical issue, and attention has been focused on thermoelectric conversion elements capable of converting heat directly into electricity as one of effective techniques for waste heat utilization.
Furthermore, as a conventional thermoelectric conversion element, for example, a thermoelectric conversion element 50 is known which has a structure including a P-type thermoelectric conversion material 51, an N-type thermoelectric conversion material 52, a lower temperature electrode 56, and a higher temperature electrode 58, as shown in FIG. 8.
In this thermoelectric conversion element 50, the two types of thermoelectric conversion materials 51, 52 are materials for energy conversion between heat and electricity, which are connected to lower temperature electrodes 56 at lower temperature joints 53b as their respective lower temperature end surfaces. In addition, the thermoelectric conversion materials 51, 52 are connected to each other at higher temperature joints 53a as their respective higher temperature end surfaces, through the higher temperature electrode 58.
Then, in this thermoelectric conversion element 50, when a difference in temperature is provided between the higher temperature joints 53a and the lower temperature joints 53b, an electromotive force is produced as a result of the Seebeck effect, and electric power is then extracted.
In the meanwhile, the electric generating capacity of a thermoelectric conversion element is determined by thermoelectric conversion characteristics of the material and the difference in temperature provided to the element, and also affected significantly by the occupancy (the ratio of the area occupied by the thermoelectric conversion material section in a plane perpendicular to the direction of the difference in temperature caused in the thermoelectric conversion element) of the thermoelectric conversion material, and the increased occupancy of the thermoelectric conversion material allows the electric generating capacity of the thermoelectric conversion element per unit area to be increased.
However, in the case of the structure of a conventional example as in the thermoelectric conversion element 50, there is a limit to the increase in the occupancy of the thermoelectric conversion materials, because a void layer for insulation is provided between the two types of thermoelectric conversion materials 51, 52.
Thus, for example, as shown in FIGS. 9(a) and (b), a thermoelectric conversion module has been proposed in which a P-type thermoelectric conversion material 51 and an N-type thermoelectric conversion material 52 are joined with an insulating layer 61 interposed therebetween, and the P-type thermoelectric conversion material 51 and the N-type thermoelectric conversion material 52 are electrically connected through an electrode 62 on the sides of the top and bottom surfaces (see Patent Document 1).
Specifically, as shown in FIG. 9(b), the P-type thermoelectric conversion material 51 and the N-type thermoelectric conversion material 52 are joined at sides (joint surfaces) thereof with the insulating layer 61 interposed therebetween, while the side of the top surface has a carbon electrode 71 provided thereon, with a nickel based wax 72 and a molybdenum electrode 73 provided sequentially on the carbon electrode 71, in such a way that the P-type thermoelectric conversion material 51 and the N-type thermoelectric conversion material 52 are electrically connected to each other by the electrode 62 composed of the carbon electrode 71, the nickel based wax 72, and the molybdenum electrode 73.
Furthermore, as the material constituting the insulating layer 61, an electrically insulating material is used which has ceramic particles dispersed in a glass matrix.
The thus configured thermoelectric conversion module has the P-type thermoelectric conversion material 51 and the N-type thermoelectric conversion material 52 joined with the insulating layer 61 interposed therebetween, in such a way that there is no space between the both thermoelectric conversion materials, thus has an increased occupancy of the thermoelectric conversion materials, and can improve the electric generating capacity per unit area.
Patent Document 1: Japanese Patent Application Laid-Open No. 2000-286467
However, in the case of joining the P-type thermoelectric conversion material 51 and the N-type thermoelectric conversion material 52 with the use of the electrically insulating material of the ceramic particles dispersed in the glass matrix as in the case of the conventional thermoelectric conversion module, when the glass component constituting the glass matrix has conductivity due to impurities, or comes to have conductivity by diffusion of the constituents of the thermoelectric conversion materials in a firing step, there is a problem of insufficient insulation between the P-type thermoelectric conversion material and the N-type thermoelectric conversion material, thereby leading to degradation of characteristics.